The main traditional industrial use for calcium has been in its carbonate form (CaCO3) as a filler for paper and rubber. Recently its use has risen as a filler for plastics and paints in the form of increasingly smaller particles in the sub nanometer range in order to provide improved characteristics to its hosts resulting from the diminished size of the calcium material. In some instances the improvements to the host have been sufficiently pronounced to warrant dramatic increase in the price of the calcium material. For use as a filler the calcium material attributes of main concern to the buyer are particle size, uniformity and packing density which are the chief controls of performance. Calcium is also widely utilized as a soil supplement or fertilizer and to a lesser extent in pharmaceuticals.
Calcium occurs mainly as calcium carbonate referred to as limestone, or in the form of calcium magnesium carbonate referred to as dolomite. Both are marine deposits usually of substantial size. They are generally mined on a large scale by open pit and the individual components selectively recovered by mechanical or chemical means.
The chief form of manganese ore is as manganese dioxide (pyrolusite), MnO2. Deposits are usually as veins recovered by underground mining. The main use of manganese is as a steel alloy. It is recovered in large pieces, crushed to useable size and fed with iron ore into blast furnaces to make steel. In this way it does not command a very high price. It also has some chemical uses. Recently manganese has become increasingly important however as the principal component in rechargeable batteries utilized for a growing variety of industrial, electronic and power equipment extending to automobile batteries with concomitant increase in price. The desired form for batteries is as lithiated manganese dioxide (Li Mn2O4) referred to as LMD.
Pyrolusite can and is being used for this purpose. It is mined, crushed, dissolved into a solution and electrochemically recovered as electrolytic manganese dioxide (EMD). This must then be re-crushed and treated with a lithium compound at high temperature to form LMD. This process is a complicated and costly procedure. In addition it has been found that the resulting LMD frequently contains minute metal particles that apparently could cause the batteries made from it to short circuit. The particles might not be removed electromagnetically nor can their presence be completely identified by scanning electron micrograph (SEM). The solution to the problem so far has been to test all new batteries employing LMD and discard those that are found to be faulty. Alternatively multiple treatment steps may be utilized to purify MnO2 as a precursor material.
We have found to our surprise that manganese as carbonate veins that cannot be economically mined normally but which exist in geology with sufficient permeability can be advantageously recovered by in situ leach mining (ISLM) as leachate producing high purity LMD. Such solution is free of metal particles because it does not entail conventional mining or crushing and thus have the possibility of causing electrical short circuit problems in batteries for which it is employed. Moreover the presence of calcium carbonate with the manganese that normally occurs in such settings offers the opportunity for its recovery as well with the manganese thereby further increasing the economics of the process.
A patent search disclosed two methods, one proposed by Geisler in U.S. Pat. No. 5,523,066 and the other by Turner in U.S. Pat. No. 6,726,828, that describe use of ISLM utilizing a mixture of acetic acid and hydrogen peroxide (for sulphide oxidation) to recover Ca, Mn, Pb and Zn as a combined leachate from a permeable geological host. Both methods rely on a separate oxidant (i.e. peroxide) and take no precautions against progressive subsurface blockage of flow-channels preventing further leaching. Neither of these methods suggest what is proposed herein.
Another patent noted (U.S. Pat. No. 4,500,398) treated sulfide ore with fluosilicic acid plus an oxidizing agent, to release the metal values.